Synthesis of Graphene-like Films by Electrochemical Reduction of

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Synthesis of Graphene-like Films by Electrochemical Reduction of Polyhalogenated Aromatic Compounds and their Electrochemical Capacitor Applications Züleyha Kuda#, emre gür, and Duygu Ekinci Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01177 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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Synthesis of Graphene-like Films by Electrochemical Reduction of Polyhalogenated Aromatic Compounds and their Electrochemical Capacitor Applications Züleyha Kudaş†, Emre Gür‡ and Duygu Ekinci†* †

Department of Chemistry and ‡Department of Physics, Faculty of Sciences, Atatürk University, 25240 Erzurum. Turkey. Corresponding Author: Tel: +90-442-2314387, Fax: +90-442-2360948, Email address: [email protected] (Duygu Ekinci)

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ABSTRACT

Graphene is a promising two-dimensional nanomaterial for many applications due to its exciting properties. In the last decade, a variety of techniques ‒each with its own set of advantages and disadvantages‒ have been developed to prepare graphene, and there are ongoing efforts to improve these techniques and to reveal new approaches. Here, we describe a simple and low‒ cost process for the bottom‒up synthesis of graphene‒like films. This new methodology involves a two‒step procedure: (i) formation of polyaromatic ring structures by the repeated covalent coupling of aryl radicals generated from electrochemical reduction of polyhalogenated aromatic compounds in aprotic solvent, and (ii) production of carbon networks by heating of polyaromatic surface films. Accordingly, polymeric films were prepared on the electrodes by electrochemical reduction of polyhalogenated compounds such as hexafluorobenzene (HFB), hexachlorobenzene (HCB) and hexabromobenzene (HBB), and then polymer films were annealed at 400 ○C for 30 min. The structure and surface characteristics of electrodeposited carbon films under self‒ and thermal‒annealing conditions were studied by spectroscopic and morphological techniques. Also, the capacitance performance of the films was evaluated by means of cyclic voltammetry, galvanostatic charge‒discharge, and electrochemical impedance spectroscopy. Results indicate that graphene‒like carbon films can be achieved by use of the electrochemical approach under mild conditions without expensive equipment, and also that these carbon materials are very promising for low‒cost energy‒storage devices.


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INTRODUCTION Graphene, a two‒dimensional monolayer of sp2‒bonded carbon atoms, has recently attracted tremendous attention owing to its large specific surface area and high mechanical strength as well as excellent thermal and electrical conductivities.1,2 This unique material is a promising candidate for many potential applications that range from batteries to optically transparent electrodes to a hydrogen‒storage medium.3-5 Since the production of monolayer graphene by mechanical cleavage of graphite in 2004,6 many approaches have been developed to prepare graphene, including chemical vapor deposition (CVD) of hydrocarbons over a metal catalyst,7,8 epitaxial growth by high-temperature treatment of silicon carbide (SiC) wafers,9 intercalation of graphite10 and chemical11/electrochemical12-14 reduction of exfoliated graphene oxide (GO). Although all these techniques offer a convenient way to produce graphene, they still have several limitations such as low production yields, high manufacturing costs, and high processing temperature. Therefore, there have been significant and sustained efforts dedicated to exploring new procedures for cost‒effective mass production of high‒quality graphene. Recently, several research studies have indicated that graphene and graphene‒like carbon‒based structures can be generated by a chemistry‒driven bottom‒up approach from polyhalogenated aromatic compounds.15-26 Most of these studies are based on surface‒ confined Ullmann polymerization followed by thermal dehydrogenation.16-24 This polymerization reaction involves aryl‒aryl coupling of aromatic radicals generated through thermal activation of aryl halides adsorbed onto single crystal metal surfaces. The Ullmann polymerization reaction has emerged as an effective route to construct two‒dimensional conjugated polymer networks27-29 and graphitic nanostructures16-21. However, this procedure


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requires reaction temperatures above 300 K in ultra‒high vacuum (UHV) environments for the activation of polymerization. In this context, electrochemical polymerization offers unique advantages such as simplicity of the set‒up, low deposition temperature, low cost and short processing times, when compared to the Ullmann method. In this study, we present an electrochemical synthesis route followed by thermal annealing to obtain the resulting graphene‒like films (Scheme 1). For this purpose, the polymer films were first prepared on gold and silicon electrode surfaces by electrochemical reduction of polyhalogenated benzene rings such as hexafluorobenzene (HFB), hexachlorobenzene (HCB) and hexabromobenzene (HBB). Next, these films were subjected to thermal annealing to form carbon networks.

Scheme 1. Schematic representation of electrochemically induced formation of graphene‒like films from polyhalogenated aromatic compounds. Polyhalogenated aromatic compounds bearing two or more halogen substituents are known as persistent organic pollutants (POPs) that are highly toxic to plants, animals and humans. In recent years, because of increasing contamination of the environment, great attention has been paid to effectively convert these compounds into less harmful substances. For the destruction of halogenated aromatics, various methods such as biodegradation,30,31 photocatalytic oxidation32,33 and high‒temperature hydrogenation34 are used. Among them, electrochemical reduction under


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direct35-42 or mediated43-47 electron transfer is known to be one of the most effective methods for the dehalogenation of aryl halides. Direct electrochemical reduction of aryl halides (ArX) in aprotic organic solvents is initiated by formation of an anion radical (ArX•-) (Eq. 1), which quickly decays to give an aryl radical (Ar•) and a halide ion (X-), as in Eq. 2. ArX + e ↔ ArX .

(1)

ArX . → Ar . + X

(2)

Aryl radicals are very reactive and are reduced more easily than the starting materials. The reduction of the radicals occurs through electron transfer either under homogeneous (Eq. 3) or heterogeneous (Eq. 4) conditions to form the aryl anions (Ar-). As soon as it is produced, the aryl anion undergoes rapid protonation by residual water or some other proton sources (the solvent or the quaternary ammonium cation of the supporting electrolyte) to form the final product (ArH) (Eq. 5). Ar . + ArX . ↔ Ar + ArX

(3)

Ar . + e ↔ Ar

(4)

Ar + H O⁄SH ↔ ArH + OH ⁄S

(5)

Alternative reaction pathways such as polymerization may also be considered for the aryl radical intermediates generated by electrochemical reduction of aryl halides. Extensive investigations in polymer chemistry revealed that a radical is able to combine with another radical to minimize its activity, resulting in formation of a dimer. By the continuation of this process, the polymer chain consisting of the same repeating units is propagated. Such a probability for polyhalogenated aromatic compounds has been investigated theoretically, and it was asserted that this strategy is not experimentally valid since the aryl radical is more easily


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reducible than the starting halide.48 Also, the maximum experimental yields for polymerization was emphasized to be in the range of 7%, although the growth of a polymer film on the electrodes during the reductive dehalogenation process was pointed out in several studies.49,50 Taking inspiration from this small possibility, we aim to produce polymeric carbon thin films via the coupling of phenyl radicals as a preliminary stage of graphene‒like structures. In this manner, we introduce a new approach to generate one of the most fascinating materials in recent years while converting aryl halides into less hazardous substances. Films prepared through this approach were characterized by Raman spectroscopy, X‒ray photoelectron spectroscopy (XPS), X‒ray diffraction spectroscopy (XRD), scanning electron microcopy (SEM), scanning tunneling microscopy (STM), and transmission electron microscopy (TEM). Applications of the carbon films for the fabrication of the electrochemical capacitors were also investigated by electrochemical techniques. Results suggest that this strategy could be a simple and general way to prepare graphene‒like carbon films. EXPERIMENTAL SECTION Hexafluorobenzene (HFB), hexachlorobenzene (HCB), hexabromobenzene (HBB) and other chemicals were used as received from Sigma‒Aldrich. Acetonitrile (HPLC grade) was purified by drying with calcium hydride, followed by distillation from phosphorus pentoxide. It was kept over molecular sieves (3 Å, Merck) in order to eliminate its water content as much as possible. N,N‒dimethylformamide (DMF, Merck) was kept over anhydrous Na2CO3 for several days and then purified by distillation under reduced pressure. Tetrabutylammonium tetrafluoroborate (TBABF4, Aldrich) was used after recrystallization. Aqueous solutions were prepared using water deionized to 18 MΩ cm−1 with a Barnstead Nanopure water purification system. Au wire


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(0.762 mm diameter, 99.999% purity) and Au foil (0.127 mm diameter, 99.99% purity) were purchased from Alfa Aesar. Silicon wafers (100 mm diameter, 500 µm thickness, , p‒type boron, single-side polished, 0.010-0.020 Ω-cm resistivity) were purchased from Topsil Semiconductor Materials A/S. The voltammetric studies were performed on a Bioanalytical Systems BAS‒100B electrochemical analyzer (Bioanalytical System Inc., Lafayette, IL, USA). A single‒ compartment electrochemical cell was used with a three‒electrode configuration. Gold and silicon were used as working electrodes. Platinum wire (BAS Model MW‒1032) and Ag/AgCl/KCl (3.0 M) (BAS Model MF‒2078) electrodes were used as counter and reference electrodes, respectively. All potentials were reported versus this reference electrode at room temperature. During the electrochemical measurements, the solutions were deaerated with high‒ purity nitrogen and kept in a nitrogen atmosphere. The gold electrodes used for electrochemical measurements were prepared as described earlier by Hamelin.51 First of all, Au wire was cleaned by sequential rinsing with distilled water, piranha solution (3:1, H2SO4, 30% H2O2), distilled water, and acetonitrile. Caution: Piranha is a vigorous oxidant and should be used with extreme caution! After being dried in nitrogen gas stream, the clean Au wire was melted in a H2/O2 flame to form a 1.5–2.5 mm diameter droplet at the end of the wire, and then the droplet was annealed in the flame. Finally, the surface of Au electrode containing some elliptical (111) facets was tested electrochemically in 1 M H2SO4 solution. Polycrystalline gold plates used for spectroscopic experiments were cleaned by immersion in piranha solution for 10 min, followed by a 1 min rinse in water. Afterwards, the Au substrates were electrochemically cleaned by cycling the electrode potential between 0.0 and 1.6 V vs. Ag/AgCl/KCl (3.0 M) in 1 M H2SO4 solution until stable voltammograms corresponding to


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an unmodified gold electrode surface were obtained, and then washed with deionized water and acetonitrile, and dried in nitrogen gas stream. Silicon wafers were cleaned in a piranha solution (3:1, H2SO4, 30% H2O2) for 30 min at 100 ˚C. After cleaning and extensive water rinse, the Si electrodes were immersed for 1.5 min in 2.5% aqueous HF solution, and then washed with distilled water and acetonitrile, and dried in nitrogen gas stream. The formation of polymer films on the working electrodes was achieved via electrochemical reduction of aromatic halides. The solutions of HFB and HCB (1 mM) were prepared by dissolving their corresponding amounts in acetonitrile containing 0.1 M TBABF4. On the other hand, the HBB (1 mM) was dissolved in freshly distilled DMF with 0.1 M TBABF4 due to its low solubility in acetonitrile. Afterwards, the electrodes were immersed in the electrolyte solutions, and the modification of electrode surfaces was achieved by repetitive potential cycling at 0.1 V s−1 scan rate. For the STM measurements, the polymeric surface films were obtained by running CV curves of three aromatic halides within their corresponding potential window for 10 cycles, whereas for the other techniques unless stated otherwise, the films were produced by running hundred CV cycles at proper negative potentials. The resulting film modified electrodes were thoroughly rinsed with deionized water and acetonitrile to remove physically adsorbed species. After the electrodeposition, the electrodes modified by polymeric films were introduced into a tube furnace (Protherm Furnaces, PTF 12/50/450) and then annealed at different temperatures (200, 400 and 700 ˚C) for 30 min under a flowing argon atmosphere. The capacitive performances of the modified electrodes were measured on a Gamry Reference 600 electrochemical workstation with cyclic voltammetry (CV) and galvanostatic charge‒ discharge (GCD) functions using a three electrode cell in 1 M Na2SO4 aqueous solution. The


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specific capacitances of the electrodes were calculated according to the following equations (Eqs. 6,7):

C, = ×∆× (6) ×∆!

C, = ×∆

(7)

where Cs,CV and Cs,GCD refer to the specific capacitance (F g-1) obtained from CVs and GCDs, respectively. I is the capacitive current (A); ∆V is the potential window of the charge and discharge (V); ν is the scan rate (V s-1); ∆t is the charge or discharge time (s); and m is the total mass of the active materials (g). Electrochemical impedance spectroscopy (EIS) was also conducted on the Gamry Reference 600 electrochemical system, and the frequency range was set to 10-106 Hz. Raman spectra were obtained on an inVia Reflex MicroRaman spectrometer (Renishaw) using the exciting beam of a Nd-YAG laser (532 nm). The powder X-ray diffractograms (XRD) of prepared films were recorded using a Rigaku Miniflex powder X-ray diffractometer with Cu Kα radiation (λ= 1.5405 Å). The structural interlayer spacing values (d002), the crystallite sizes (Lc) and the layer numbers (Nave) of the samples were estimated using the following equations (Eqs 8-10): $

d## = (8) %&' *$

L) = (9) +),' N./0 =

12 334


(10)

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where λ is the wavelength of the X-ray radiation, θ is the angular position of the 002 peak, B is the full-width at half maximum intensity (fwhm) of the 002 carbon peak and K is a constant (0.89). X-ray photoelectron spectroscopy (XPS) experiments were carried out using a SPECS (Berlin, Germany) spectrometer operated in constant analyzer energy mode (20 eV). A monochromated Al Kα source (hν=1486.6 eV) was employed at a power of 600 W. The data were obtained at room temperature, and typically the operating pressure in the analysis chamber was below 1×10−9 Torr. The binding energy scale was calibrated with respect to the C1s binding energy of 284.6 eV. The peaks were fitted using Gaussian profiles. Peak areas were obtained by numerical integration of the component areas. Scanning tunneling microscopy (STM) measurements were acquired in ambient conditions, with a Molecular Imaging Model PicoScan instrument. STM tips were mechanically cut Pt–Ir wire. The single crystal gold (Au(111)) electrodes were used for STM measurements. Scanning electron microscopy (SEM) analyses were recorded using a FEI Inspect S50 SEM equipped with an Energy Dispersive X‒ray analyzer (EDXA). Transmission electron microscopy (TEM) analyses were performed on a JEOL JEM 2100F electron microscope 200 kV. The sheet resistances of the films were measured by a four‒wire sensing system in van der Pauw configuration (Keithley 2635 system). To characterize the species produced during electrolysis of aryl halides, gas chromatography‒ mass spectrometry (GC‒MS) analyses were carried out on a Thermofinnigan Trace GC/Trace DSQ/A1300 (E.I. Quadrapole) gas chromatography system equipped with a SGE-BPX5 MS fused silica capillary column (30m×0.25mm internal diameter × 0.25 µm film thickness). For


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GC–MS detection, an electron impact ionization system with ionization energy of 70 eV was used. Carrier gas was helium at a flow rate of 1mL/min. RESULTS AND DISCUSSION Electrochemical reduction of polyhalogenated benzene rings. Direct electrochemical reduction of HFB, HCB and HBB in a non‒aqueous medium was investigated by use of cyclic voltammetry (CV), differential pulse voltammetry (DPV) and chronocoulometry (CC). The solutions of HFB and HCB were prepared in acetonitrile containing 0.1 M TBABF4. On the other hand, the HBB was dissolved in DMF with 0.1 M TBABF4 due to its low solubility in acetonitrile. Figure 1 shows cyclic and differential pulse voltammograms obtained with gold electrodes during cathodic reduction of the aromatic halides. As shown, HCB (Figure 1a) and HBB (Figure 1b) exhibit six irreversible cathodic peaks that are attributed to successive reductive cleavage of the aryl carbon‒halogen bonds. On the other hand, HFB displays three irreversible cathodic waves by one very prominent peak at -2.31 V vs. Ag/AgCl/KCl (3.0 M) (Figure 1c). Also, the cathodic peak potentials of HFB are significantly more negative than those of HCB and HBB (Table 1). The large negative shift of about 1500 mV in reduction peak potentials in going from HBB to HFB implies a higher activation energy barrier for C‒F bond cleavage.52


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Figure 1. Cyclic and differential pulse voltammograms of HCB (a), HBB (b) and HFB (c) at 1 mM concentration. The curves were recorded in acetonitrile (curves a,c) and in DMF (curve b) containing 0.1 M TBABF4. Experimental conditions: CV, scan rate 0.1 V s-1; DPV, pulse amplitude 50 mV, pulse width 50 ms, pulse period 300 ms, scan rate 25 mV s-1.

Table 1. CV and DPV peak potentials for reduction of polyhalobenzenes at gold electrodes. Peak Potential/mV (CV/DPV) compound

EpI

EpII

EpIII

EpIV

EpV

EpVI

HBB

-720

-1070

-1395

-1620

-1920

-2310

-630

-1010

-1280

-1550

-1850

-2180

-1395

-1570

-1810

-2095

-2366

-2630

-1315

-1555

-1780

-2020

-2260

-2560

-2260

-2625

-2795

-

-

-

-2210

-2450

-2730

HCB

HFB


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Reduction of organic halides at inert electrodes is a dissociative electron transfer (DET) leading to the scission of the carbon‒halogen bond, and it is well known that there are two possible reaction pathways after the injection of one electron into an organic halide: (i) a mechanism in which electron transfer and bond rupture occur in a stepwise manner with formation of an intermediate radical anion, and (ii) a concerted mechanism yielding directly a radical and a halide ion.53-55 Most often, the transfer coefficient (α) can be used as a diagnostic criterion in inferring whether the reaction mechanism is stepwise or concerted. In the stepwise mechanism, the first step requires a low energy barrier, and hence the expected value of α is quite close to 0.5. Conversely, in the case of the concerted mechanism, the heterogeneous electron transfer is the rate-determining step because a bond is broken in conjunction with electron transfer. Therefore, the concerted process needs to overcome relatively high energy barrier, and thus the value of α is significantly smaller than 0.5. The magnitude of the transfer coefficient is generally obtained from the variation of the peak potential with scan rate according to Eq. 11, 567 58,9:

=−

Synthesis of Graphene-like Films by Electrochemical Reduction of

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